Method and system for communication in multi-user multiple-input-multiple output wireless networks

ABSTRACT

Wireless communication in a wireless system using a multiple user transmission opportunity is provided. The data blocks are organized in order of transmission priority based on access categories. Contention for access to the communication medium during a transmission opportunity period is based on a backoff timer of each access category and the transmission priority. Upon successful contention for a transmission opportunity period, during the transmission opportunity period, a data block of a primary access category is wirelessly transmitted from the wireless station to one or more primary destination wireless receivers. Simultaneously, a data block of a secondary access category is wirelessly transmitted from the wireless station to one or more secondary destination wireless receivers. Contending for the transmission opportunity period includes each access category contending for access to the wireless communication medium and a secondary access category selectively invoking communication medium access backoff based on one or more backoff events.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/177,386 filed Jul. 6, 2011, which claims priority from U.S.Provisional Patent Application Ser. No. 61/362,280 filed Jul. 7, 2010,incorporated herein by reference.

FIELD

The present disclosure relates generally to the field of wirelessnetworks, and in particular, to wireless networks where multipleantennas are used to transmit multiple downlink traffic streams tomultiple receiver stations simultaneously.

BACKGROUND

In a typical wireless network utilizing a coordination function forcoordinating transmissions among wireless stations, such a coordinationfunction may be implemented in one of the wireless stations such as awireless access point (AP) functioning as a coordinator. The wirelessstations may communicate via directional transmissions using sectorantennas and beam-forming antenna arrays. The coordinator may useomnidirectional transmissions for broadcasts to all wireless stations inall directions (e.g., 360 degrees range).

Alternatively, the coordinator may use quasi-omnidirectionaltransmissions for broadcasts to a wide range, but not necessarily in alldirections. In many wireless area networks (WLANs) such as thoseaccording to IEEE 802.11 standards, a coordinator is used ininfrastructure mode for providing contention-free access to a wirelesscommunication medium to support Quality of Service (QoS) for certainapplications.

BRIEF SUMMARY

Embodiments provide wireless communication in a wireless network using amultiple user transmission opportunity. One embodiment comprisesmaintaining data blocks at a wireless transmitting station fortransmission to multiple wireless receiving stations over a sharedwireless communication medium, wherein the data blocks are organizedbased on access categories.

Access to the communication medium includes contending for access to thewireless communication medium. Upon successful contention for atransmission opportunity period, during the transmission opportunityperiod, one or more data blocks of a primary access category aretransmitted from the transmitting station to one or more primarydestination wireless receiving stations, through one or more sets ofspatial streams, over the wireless communication medium. Simultaneously,one or more data blocks of a secondary access category are transmittedfrom the transmitting station to one or more secondary destinationwireless receiving stations, through other sets of spatial streamswhenever possible, over the wireless communication medium.

Contending for the transmission opportunity period comprises each accesscategory contending for access to the wireless communication medium, anda secondary access category selectively invoking communication mediumaccess backoff based on one or more backoff events.

These and other features, aspects and advantages will become understoodwith reference to the following description, appended claims andaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary block diagram of a wireless systemimplementing multi-user transmit opportunity (MU-TXOP) for multi-usermultiple-input-multiple-output (MU-MIMO) communication.

FIG. 2A shows an example Access Category (AC) scenario at an accesspoint (AP) station for simultaneous frame transmission to multipledestination stations during a MU-TXOP over a wireless communicationmedium.

FIG. 2B shows an example timing diagram of a frame exchange sequencecorresponding to FIG. 2A.

FIG. 3A shows an exemplary diagram of a wireless system implementingMU-TXOP for MU-MIMO communication, according to an embodiment of thepresent invention.

FIG. 3B shows an exemplary timing diagram of a wireless channel accessand transmission sequence in a MU-TXOP for MU-MIMO communication.

FIG. 4 shows an exemplary flowchart of a frame exchange process usingMU-TXOP in a wireless system implementing MU-MIMO.

FIG. 5 shows an exemplary timing diagram of a wireless channel accessand transmission sequence in a MU-TXOP for MU-MIMO communicationaccording to the process in FIG. 4.

FIG. 6 shows a flowchart for a backoff process in response to a backoffevent, according to an embodiment of the invention.

FIG. 7 shows a wireless communication system with overlapping wirelessnetworks, implementing backoff procedures, according to an embodiment ofthe invention.

FIG. 8 is an exemplary high level block diagram showing an informationprocessing system comprising a computer system useful for implementingdisclosed embodiments.

DETAILED DESCRIPTION

Embodiments of the present invention provide a method and system forsimultaneously transmitting multiple downlink spatial streams tomultiple receiver wireless stations during a multi-user transmitopportunity (MU-TXOP) over a wireless communication medium.

Generally in the absence of a coordinator, contention-free access to awireless communication (e.g., a radio frequency (RF) channel) may beimplemented using announcement or information exchange among wirelessstations in a network to negotiate/reserve the use of the communicationmedium. For example, IEEE 802.11e Enhanced Distributed Channel Access(EDCA) provides QoS support for certain applications using announcementor information exchange. EDCA defines four Access Categories (ACs) andintroduces service differentiation such that certain data traffic useshigher priority parameters to contend for the communication medium.

Further, a frame structure may be used for data transmission betweenwireless stations such as a transmitter station and a receiver station.In one example, a frame structure in a Media Access Control (MAC) layerand a physical (PHY) layer is utilized, wherein in a transmitterstation, a MAC layer receives a MAC Service Data Unit (MSDU) andattaches a MAC header thereto, in order to construct a MAC Protocol DataUnit (MPDU). The MAC header includes information such as a sourceaddress (SA) and a destination address (DA). The MPDU is a part of a PHYService Data Unit (PSDU) and is transferred to a PHY layer in thetransmitter to attach a PHY header (i.e., PHY preamble) thereto toconstruct a PHY Protocol Data Unit (PPDU). The PHY header includesparameters for determining a transmission scheme including acoding/modulation scheme. The PHY layer includes transmission hardwarefor transmitting data bits over a wireless link. Before transmission asa frame from the transmitter station to the receiver station, a preambleis attached to the PPDU, wherein the preamble can include channelestimation and synchronization information.

EDCA allows contention for transmission opportunities (TXOPs), wherein aTXOP is a time interval when a QoS wireless station (STA) may initiateframe transfer on the wireless medium (e.g., wireless channel). The TXOPmay be assigned to the wireless station by a coordinator, or thewireless station may obtain the TXOP by successfully contending for thewireless channel.

Conventionally, a single user TXOP (SU-TXOP) defined in IEEE 802.11standards is utilized per AC. As such, a SU-TXOP obtained by a stationonly sets the Network Allocation Vector (NAV) timer for a specific AC(used to contend for the TXOP), to idle during the SU-TXOP period. NAVsfor other Access Categories (ACs) of the same station are set to busy.NAV is a counter maintained by each station, indicating the time toelapse until the channel is free again, such that a station cannottransmit until its NAV is zero. An EDCA TXOP is granted to EnhancedDistributed Channel Access Function (EDCAF) when the EDCAF determinesthat it may initiate a frame exchange sequence. During an EDCA SU-TXOP,a wireless station may initiate multiple frame exchange sequences totransmit MAC Management Protocol Data Units (MMPDUs) and/or MSDUs onlywithin the same AC. Internal contention among frames belonging todifferent ACs allows only one AC to win the internal competition for theTXOP.

Disclosed embodiments herein provide a MU-TXOP mechanism for a wirelesscommunication system such as a wireless network to support multipledownlink traffic streams to multiple receiver wireless stationssimultaneously during the MU-TXOP (i.e., a shared transmissionopportunity period). According to an embodiment, contention among accesscategories is resolved, in some cases, by allowing access categories toshare a transmission opportunity period.

One embodiment comprises maintaining data blocks at a wirelesstransmitting station for transmission to multiple wireless receivingstations over a shared wireless communication medium, wherein the datablocks are organized based on access categories.

Access to the communication medium includes contending for access to thewireless communication medium. Upon successful contention for atransmission opportunity period, during the transmission opportunityperiod, one or more data blocks of a primary access category aretransmitted from the transmitting station to one or more primarydestination wireless receiving stations, through one or more sets ofspatial streams, over the wireless communication medium. Meanwhile, oneor more data blocks of a secondary access category are transmitted fromthe transmitting station to one or more secondary destination wirelessreceiving stations, through other sets of spatial streams wheneverpossible, over the wireless communication medium.

Contending for the transmission opportunity period comprises each accesscategory contending for access to the wireless communication medium, anda secondary access category selectively invoking communication mediumaccess backoff based on one or more backoff events.

In one embodiment the invention provides a method and system forcommunication in a wireless network such as a Wireless LAN (e.g., IEEE802.11), where multiple antennas are used to transmit multiple downlinktraffic streams from a wireless transmitter station to multiple wirelessreceiver stations simultaneously over a wireless communication medium.In one embodiment, the wireless communication medium comprises awireless RF channel. Embodiments of said method and system providechannel access rules, including backoff procedures, when transmitopportunities can be shared by multiple Access Categories.

According to an embodiment, data blocks are maintained at a wirelesstransmitting station for transmission to multiple wireless receivingstations over a wireless medium. The data blocks are organized accordingto their access category, which assigns different transmissionpriorities to the data blocks. EDCAF of each AC of the transmittingstation contends for access to the wireless medium based on its backofftimer value and transmission priority.

In one embodiment, an access category is divided into two classes when aTXOP is shared: a primary AC and a secondary AC. The destinationwireless stations are divided into two classes, the primary destinationand the secondary destination. When contending for transmissionopportunity, only the EDCA parameters of the primary AC are used, notthe combination of all ACs that have buffered data, or the EDCAparameters of the highest priority AC.

Upon successful contention for a transmission opportunity period, duringthe transmission opportunity period, one or more data blocks of aprimary access category are transmitted from the transmitting station toone or more primary destination wireless receiving stations through oneor more sets of spatial streams over the wireless communication medium.Simultaneously, one or more data blocks of one or more secondary accesscategories are transmitted from the transmitting station to one or moresecondary destination wireless receiving stations, through one or moreother sets of spatial streams over the wireless communication medium.Multiple frame transmissions are possible provided that the TXOP limitof the primary AC has not been reached.

In the description herein, primary AC comprises an AC that wins the TXOPfor channel access after both external and internal competition. Therecan be only one primary AC at any moment. Secondary AC comprises an ACwhich does not win a TXOP but wishes to share the TXOP obtained by theprimary AC for simultaneous transmissions. There can be multiplesecondary ACs at any moment. Primary destinations comprise destinationstargeted by frames belonging to the primary AC. There can be one or moreprimary destinations at any moment. Secondary destinations comprisedestinations targeted by the frames belonging to secondary ACs. Therecan be one or more secondary destinations at any moment.

According to an embodiment, internal contention among access categoriesis resolved by allowing secondary ACs to share a TXOP. In oneembodiment, the invention provides a backoff procedure for anEDCA-based, Multi-User MIMO equipped Wireless LAN. In oneimplementation, a backoff procedure for EDCA-based channel accessprovides backoff rules when a TXOP (e.g., MU-TXOP) is shared by multipleACs.

In one embodiment, the present invention provides a backoff process forMU-TXOP. In one implementation, for a secondary AC if the communicationmedium is physically busy, the transmitter station should performbackoff as is routine (e.g., as in conventional IEEE 802.11 standards).However, if the communication medium is virtually busy, in thetransmitter station the secondary AC checks whether any other AC of thetransmitter station has won the TXOP and whether sharing the TXOP ispossible. If sharing of TXOP is granted to this AC, the AC will thentransmit and will not invoke backoff, otherwise, the secondary ACinvokes backoff as is routine.

Because the secondary AC is sharing the TXOP obtained by the primary AC,its transmission of data frames is limited by the length of TXOPobtained by the primary AC. A sharing rule requires that the time usedto transmit the frames of the secondary AC cannot exceed the time usedto transmit the frames of the primary AC. Therefore, the length of theshared TXOP may or may not fulfill the requirement of transmitting whatthe secondary AC plans to transmit. In one implementation of theinvention, the secondary AC has not finished transmission of what itplans to transmit as if the TXOP was obtained by itself, whereby thedesired transmission is partially completed. The secondary AC may resumeits timer countdown from the previously frozen value in order to obtainits own TXOP or upon an opportunity to share again with other primaryTXOPs that will follow.

When transmission of a frame for a secondary AC fails, in oneimplementation the secondary AC invokes backoff together with other ACsthat experienced transmission failure, while in another implementationthe secondary AC may utilize Direction Based Backoff (DBB), as describedfurther below.

When a transmission attempt of an EDCAF of an AC collides internallywith another EDCAF of the AC of higher priority, if such collisioncannot be eliminated by TXOP sharing, then exponential backoff isutilized. If such collision cannot be resolved, then backoff is notinvoked.

As noted, in one embodiment, a DBB mechanism is provided forcircumstances where the transmissions are performed via directional(e.g., beamforming) transmissions. This allows the transmitting wirelessstation (sender) to identify a collision spatial area and only invokebackoff for frames sent to that specific direction/destination. Forother non-collision areas (including other directions/destinations), thesender need not invoke backoff. This allows secondary ACs to resumetheir transmission at their own pace (using a backoff timer and aContention Window (CW) before the TXOP sharing) when their intendedtransmission has been partially performed by sharing the TXOP obtainedby another AC. Direction Based Backoff allows the sender to invokebackoff in a collision area near one or more specific receiver(s) andstill be able to send to areas that are not experiencing collisionwithout invoking backoffs. This increases the network throughput.

Embodiments of the invention are useful with WLANs (such as IEEE 802.11ac) for Downlink (DL) Multi-User MIMO (DL MU-MIMO) communication.Embodiments of the invention allow multiple traffic streams to betransmitted from a wireless transmitting station (sender) to differentwireless receiving stations simultaneously via multiple spatial streamsthrough the use of beamforming technology.

FIG. 1 shows an exemplary wireless network 10. The wireless networkcomprises a WLAN comprising multiple wireless stations. A wirelessstation 11 (i.e., STA-A) comprises an AP having a PHY layer 14 and a MAClayer 12 implementing an EDCA MU-TXOP module (i.e., channel accessmodule) 16. Several traffic streams (or queues) of data frames (packets)in multiple different access categories ACs at the AP station 11 are fortransmission to multiple receiver wireless stations 13. Each wirelessstation 13 comprises a MAC layer 13M and a PHY layer 13P.

In this example, there are three traffic streams (or queues) of dataframes (packets) in three different access categories AC0, AC1, and AC2at the AP station 11 for transmission to receiver wireless stations 13(i.e., STA-B, STA-G and STA-D), respectively. AC0 is a primary AC, andAC1 and AC2 are secondary ACs.

MU-TXOP according to the invention is useful in both EDCA and hybridcoordination function controlled channel access (HCCA). The exampleembodiments described herein are for EDCA, wherein a MU-TXOP is assignedto a MU-MIMO wireless station such as the AP station 11 in FIG. 1.

In one embodiment, the EDCA MU-TXOP module 16 implements three modes. Afirst mode involves initiation of the EDCA MU-TXOP which occurs when theEDCA rules permit access to the wireless communication medium (e.g.,wireless RF channel). A second mode involves sharing of the EDCA MU-TXOPwhich occurs after an EDCAF is granted the TXOP. A third mode involvesmultiple frame transmission within an EDCA MU-TXOP, which occurs when anEDCAF retains the right to access the wireless communication mediumfollowing the completion of a frame exchange sequence, such as onreceipt of multiple block acknowledgement (BA) frames from receivingwireless stations 13. The three examples EDCA MU-TXOP modes aredescribed further below.

In one embodiment, an initiation of the MU-TXOP occurs when the EDCArules permit access to the wireless communication medium. Although theremay be other factors that affect the transmission decision (such asscheduling), in general, the EDCAF of an AC of the transmitting APstation 11 is allowed to access the wireless communication medium if allthe following conditions are satisfied:

-   -   1) The backoff timer of the AC has counted down to zero when the        backoff slot boundary is reached;    -   2) The AC has higher priority than other ACs;    -   3) The AC has buffered data to transmit.

When an AC is allowed to access the wireless communication medium afterwinning both the external (with other STAs) and the internal (with otherACs of the same STA) competition, it becomes the primary AC. An EDCAMU-TXOP is granted to the EDCAF of this AC. Other ACs become secondaryACs and may be able to share the obtained EDCA MU-TXOP.

According to an embodiment, when contending with other stations fortransmission opportunity for access to the wireless communication medium(e.g., radio frequency wireless channel), the rules the AP station 11uses remain the same as in the conventional IEEE 802.11 wirelesscommunication specifications. Each AC of the AP station uses a set ofEDCA parameters of this AC (e.g., AIFS[AC]) to contend for wirelesschannel access.

The AP station 11 may not always use its highest priority AC forcontending the TXOP even though the highest priority AC may be able toshare the TXOP for simultaneous transmission. Otherwise, such a behaviorby AP station 11 in using its highest priority AC for contending theTXOP will break the fairness of EDCA access rules, and other non-APstations (such as stations 13) will have less chance and less time fortransmission.

In one embodiment, the ACs (e.g., AC0, AC1, AC2) at the AP station 11are divided into two classes: primary AC and secondary AC. A primary ACis an AC that wins both external competition among multiple STAs 13, andinternal competition among multiple ACs at the AP station 11. Theprimary AC is not always the highest priority AC (e.g., AC0 in FIG. 1)even though the highest priority AC has a better chance to become aprimary AC. The primary AC is the “actual holder” of the MU-TXOP.Secondary ACs include the remaining ACs at the AP station (e.g., AC1,AC2). The frames of the secondary ACs share the MU-TXOP with the framesof the primary AC, for essentially simultaneous transmission from the APstation 11 to multiple receiver stations 13 over the wireless channel.

The destination wireless stations 13 targeted by the frames of theprimary AC at the AP station 11 are defined as primary wireless stationdestinations. If the frames of the primary AC target more than onedestination wireless station, there will be multiple primarydestinations. The destination wireless stations 13 only targeted byframes of secondary ACs are defined as secondary destinations. When theframes of the primary AC and frames of one or more secondary ACs targetthe same destination wireless station, the destination wireless stationis still a primary destination and the frames of the secondary ACs mustyield to the frames of the primary AC.

According to an embodiment, sharing of a MU-TXOP allows secondary ACs totransmit their data frames in the TXOP period obtained by the primary ACat the AP station 11. In one embodiment, sharing takes place when thefollowing conditions are satisfied:

-   -   1) Frames for multiple STAs exist.    -   2) DL MU-MIMO transmission is possible: [0056] If, for example,        the primary AC and one or more ACs have frames targeting to the        same destination STA, then DL MU-MIMO transmission is not        possible. In this case, single user MIMO (SU-MIMO as in        conventional IEEE 802.11n standard) can be used for transmitting        the data of the primary AC at the AP station 11 and a secondary        AC cannot share the TXOP.    -   3) DL MU-MIMO transmission is appropriate:        -   In certain cases, both conditions 1) and 2) above are met,            but it is still not appropriate to use MU-MIMO protocol and            share MU-TXOP. For example, when two or more destination            STAs 13 targeted by both primary and secondary AC at the AP            station are spatially too close to each other (exposing the            stations to potential RF interference), then SU-MIMO is a            more appropriate transmission protocol at the AP station 11.

In one embodiment, sharing a MU-TXOP is performed by grouping of thewireless stations 13, wherein the primary wireless station destinationsare grouped with one or more secondary wireless station destinations.The MAC layer 12 at the AP station 11 can group one or more primarywireless station destinations with one or more secondary wirelessstation destinations in different ways. In one implementation, a GroupIDcan be assigned to a selected group of wireless station destinations andutilized to identify them. Each wireless station in the selected grouphas knowledge of the group definition (i.e., GroupID) prior to datatransmission for correct reception of the data frames at such wirelessstation destinations 13.

Once secondary wireless station destinations are selected, thecorresponding secondary ACs at the AP station 11 treat the wirelesschannel as idle to allow simultaneous transmissions.

Internal competition among the ACs at the AP station 11 for a TXOP isresolved by allowing secondary ACs to transmit in the same TXOP of theprimary AC. However, in certain cases simultaneous transmissions are notpossible. In one of such cases, frames of different ACs transmit to thesame destination wireless stations. In another such case, thetransmission time of the secondary ACs is longer than that of theprimary AC at the AP station 11, wherein the secondary AC is unwillingto fragment the frame (or it is not possible to fragment). In that case,the additional time taken by the secondary AC for transmission may leadto unfairness to other stations in the WLAN Basic Service Set (BSS).

When parallel (i.e., simultaneous) transmission is not possible at theAP station 11, a lower priority AC backs off as in the conventional IEEE802.11 standard. The primary AC may choose to use SU-MIMO for its owntransmission, after the lower priority AC backs off.

In one embodiment, a scheduler 18 at the AP station (FIG. 1) determineswhich frames (data blocks) 15 are to be transmitted to multiple receiverstations, wherein the frames are organized into ACs based on QoS rules,and arranged into queues accordingly. A manager module 19 determines theprimary AC to be used for contending for a transmission opportunity.

FIGS. 2A-2B show an example wherein data frames in different ACs at theAP station 11 (FIG. 2A) can share the MU-TXOP. In FIG. 2A, it is assumedthat AC_VI is the primary AC having two sets of MSDU frames 15 (e.g.,AC_VI(1) and AC_VI(2)) in memory buffer to wirelessly transmit, one fordestination station STA-B and the other for destination station STA-D.As such, both destination STA-B and STA-D are primary destinations.AC_VO and AC_BE are secondary ACs, and destination STA-C is a secondarydestination.

Further, AC_VO has two sets of MSDU frames for transmission, a first setAC_VO(1) for transmission to destination STA-D and a second set AC_VO(2)for transmission to destination STA-C. In addition, AC_BE has two setsof MSDU frames, a first set AC_BE(1) for transmission to destinationSTA-C and a second set AC_BE(2) for transmission to destination STA-D. Aremaining AC_BK has no frames to transmit. FIG. 2B shows a correspondingtransmission process 20 for the aforementioned MSDU frames from theprimary and secondary ACs in AP station 11 using a shared MU-TXOP over awireless communication channel to the destination wireless stations 13in a frame exchange sequence. The wireless stations 13 may transmitresponsive frames (e.g., acknowledgment (ACK) frames) back to the APstation 11 over the wireless communication medium.

The MSDU frames 15 in memory buffer at the AP station 11 may befragmented or aggregated into multiple Aggregated MAC Protocol DataUnits (A-MPDUs), each being transmitted in one downlink phase.

According to an embodiment, once a MU-TXOP is granted to the AP station11, in a frame exchange sequence with destination stations 13, theframes associated with different ACs at the AP station 11 share thatMU-TXOP for downlink transmissions to multiple wireless stations 13,wherein all the transmissions take place at the same time via differentspatial streams over the wireless communication medium. Therefore, theMU-TXOP is shared among multiple sets of spatial streams (each settargeting one destination station) that can belong to multiple ACs forthe downlink transmission from the AP station 11 to multiple destinationwireless stations 13. Unlike SU-TXOP, data frames belonging to secondaryACs that are scheduled to be transmitted with that of the primary AC usethe same TXOP for transmission at the AP station 11.

In one example in the network 10 of FIG. 1, during a MU-TXOP, multipletraffic/transmission streams/paths belonging to different accesscategories AC0, AC1, AC2 at the AP station 11 are transmittedsimultaneously over a wireless communication medium to multiple wirelessstations 13 (i.e., STA-B, STA-C and STA-D) over multiple wirelessstreams/paths (i.e., Path1, Path2, Path3). The AP station 11 implementsmulti-user multiple-input-multiple-output (MIMO) at its PHY layer 14 forsuch simultaneous wireless transmissions via multiple antennas 17 to thewireless stations 13 over the wireless communication medium.

In one embodiment, the MU-TXOP module 16 at the MAC layer of the APstation 11 comprise protocols, hardware and/or software implementations,which support downlink (DL) multi-user MIMO (DL MU-MIMO) wirelesscommunication to the wireless stations 13 over the wirelesscommunication medium. In the network 10, DL MU-MIMO wirelesscommunication allows a sender station such as the AP station 11 toobtain a MU-TXOP for simultaneously transmitting frames 15 in AC0, AC1and AC2 via multiple traffic streams to different receiving wirelessstations 13 such as STA-B, STA-C, STA-D, using directional transmissionvia the wireless communication medium. In one embodiment, multi-userdirectional transmissions using beam-forming are utilized between the APstation 11 and each of the multiple wireless stations 13 (i.e., STA-B,STA-C, STA-D). Beam-steered wireless signals comprise directional beamsignals, wherein each directional beam (i.e., path) comprises a mainlobe and side lobes.

In one embodiment, the TXOP duration is determined by the TXOP limit ofthe primary AC. At least one spatial stream set in each DL MU-MIMO PPDUcontains only MSDU(s) corresponding to the primary AC, wherein a streamset is defined as a group of spatial streams of a DL MU-MIMO PPDU thatare all intended for reception by a single recipient station. As usedherein, PPDU stands for physical (PHY) layer convergence procedure(PLCP) protocol data unit.

FIG. 3A illustrates an example downlink transmission in the wirelessnetwork 10 involving multi-user MIMO transmission of frames B, C, D fromthe AP station 11 (STA-A) to the receiver stations STA-B, STA-C, STA-Dduring a MU-TXOP, respectively, via directional transmissions over thewireless communication medium.

FIG. 3B shows a timing diagram 25 for the example communication in FIG.3A, wherein during a MU-TXOP in a downlink phase, the AP station 11(STA-A) simultaneously directionally transmits three frames B, C, D(each with a specified destination receiver station address (RA)) to thereceiver stations 13 (STA-B, STA-C and STA-D), respectively. In anuplink phase, each of the receiver stations 13 sends a blockacknowledgement (BA) to the AP station 11 sequentially using apredefined schedule, over the wireless communication medium.

During an EDCA MU-TXOP obtained by an EDCAF at the AP station 11, theMU-MIMO AP station 11 may initiate multiple frame exchange sequenceswith destination wireless stations 13 to transmit MMPDUs and/or MSDUsbelonging to different ACs. For each frame exchange sequence, there canbe multiple simultaneous spatial streams targeting different receiverstations 13, belonging to different ACs at the AP station 11.

The MU-TXOP process according to an embodiment, allows frames belongingto different ACs at the AP station 11 to be transmitted using a TXOPobtained for the frames of the primary AC at the AP station 11. In oneimplementation, frames 15 belonging to a lower priority AC at the APstation 11 can share the TXOP as long as such frames are essentially ofsimilar length as that of the primary AC. As such throughput can beincreased without negatively affecting application QoS rules for theinvolved frames 15. Using frames of similar length provides an AP theflexibility to select among the secondary AC frames for transmission.

A MU-TXOP is obtained only using EDCA parameters of the primary AC atthe AP station 11, neither a combination of all ACs involved, nor alwaysthe highest priority AC that has frames to transmit. Though a MU-TXOP isobtained using EDCA parameters of the primary AC, the MU-TXOP is sharedby multiple traffic streams from the AP station 11 to the destinationstation 13, which may or may not belong to that AC.

In one embodiment, the same GroupID for the selected destinationstations 13 is used to identify the destination stations 13 during theentire multiple frame exchange sequence between the AP station 11 andthe destination stations 13. In an alternative embodiment, GroupID maybe changed during the multiple frame exchange sequence. This may be moreefficient when the number of destination stations 13 is large (e.g.,larger than 4) and the TXOP duration is long.

In one embodiment, multiple frame exchanges may be performed between theAP station 11 and the destination stations 13 when the primary ACpossesses remaining frames to transmit in its buffer (queue). In oneembodiment, once the primary AC at the AP station completes itstransmission, the MU-TXOP terminates even if secondary ACs possesremaining frames 15 in their buffers. As such, secondary ACs cantransmit their frames as long as the primary AC has not completedtransmission of its frames at the AP station 11. Once the primary AC hascompleted transmission of its frames, the frames of secondary ACs cannotbe transmitted and need to await a next MU-TXOP. When the primary ACcompletes its frame transmission, the MU-TXOP can be terminated by theMU-MIMO AP station 11 by transmitting a contention-free end (CF-End)frame over the wireless communication medium if the remaining time issufficient for transmission of such a CF-End frame.

In one embodiment of the invention, to transmit frames 15 of differentACs from the AP station 11 to different destination station 13, theframes may be aggregated into one A-MPDU utilizing a frame aggregationprocess, and utilizing a multi-traffic block ACK (MTBA). In oneimplementation, as illustrated by the timing diagram 50 in FIG. 5, shortinterframe spaces (SIFSs) may be used to separate frames betweendownlink transmission (from AP station 11 to a destination station 13)and uplink transmission (from a destination station 13 to AP station 11)phases, as well as multiple response frames (i.e., clear-to-send or CTS)on responsive uplink transmissions. The assumptions for FIG. 5 are thesame as that for FIGS. 2A-2B. In FIG. 5, a scheduled BA scheme is usedfor the uplink phase, which does not prevent other acknowledgementsschemes to be utilized (e.g., a poll-based acknowledgement scheme).

In one embodiment, frame fragmentation or aggregation may be applied toframes for secondary ACs at the AP station 11 such that the transmissiontime of frames for secondary ACs are similar to that of the frames ofthe primary AC for each frame exchange sequence. In one embodiment ofthe invention, all stations 11 and 13 contend for transmissionopportunity again after a MU-TXOP ends.

In one embodiment, the duration of EDCA MU-TXOP is bounded by theprimary AC at the AP station 11. Such duration can be as defined by IEEE802.11 standards based on dot11QAPEDCATXOPLimit MIB variable for anaccess point. A value of 0 for EDCA MU-TXOP duration means that the EDCAMU-TXOP is limited to a single frame exchange sequence, in time domain,at any transmission rate in the operational set of the BSS. The APstation 11 may transmit multiple frames in the spatial domain tomultiple receiver stations 13, wherein each frame is carried by aspatial stream set. There can be multiple frames in the spatial domain,each for one receiver station.

Each spatial stream set contains a group of spatial streams of adownlink MU-MIMO PPDU that are all intended for reception by a singlereceiver station 13. In one embodiment, at least one stream set in eachDL MU-MIMO PPDU contains only MSDU(s) corresponding to the primary AC.This is to ensure that the AP does not transmit only frames of secondaryACs during any downlink phase.

FIG. 4 shows an exemplary flowchart of a process 30 for frame exchangebetween an AP station (such as AP station 11) and destination wirelessstations (such as stations 13) using MU-TXOP. The example timing diagramin FIG. 5 may be based on the frame sequence exchange process 30according in FIG. 4, wherein frames to destination STA-B have thehighest AC priority (primary AC). Referring to process 30 in FIG. 4, inconjunction with FIG. 5, in block 31A, a collision protection mechanismsuch as request-to-send (RTS) and CTS protocol is utilized to preventpacket collision on a shared wireless communication channel. In block31B, group definition information through management frame (if anyneeded) is provided. In block 31C, channel sounding (if any needed) isperformed.

In block 32, during an assigned MU-TXOP, the AP station 11 performsdownlink transmission of a first frame sequence for all ACs at the APstation 11 to multiple receiver stations 13 via multiple spatialstreams. In block 33, the AP station 11 receives acknowledgmentresponses (e.g., BA) from the receiver stations 13. In block 34, the APstation 11 determines if the primary AC frames have completedtransmission. If not, in block 35 it is determined if there issufficient time remaining in the MU-TXOP for remaining frames at the APstation 11. If sufficient time remains in the MU-TXOP, then in block 36,a next frame sequence of the primary AC and any remaining secondary ACs,are transmitted from the AP station 11 to receiver stations 13 viamultiple spatial streams, and the process proceeds to block 33.

If insufficient time remains in the MU-TXOP for AC frame transmission,then the process proceeds to block 37 wherein it is determined if theremaining time is sufficient for transmission of a CF-End frame. If yes,then in block 38 the MU-TXOP is terminated (truncated) by transmissionof a CF-End frame from the AP station 11 to all destination stations 13.Further, if in block 34 it is determined that the primary AC frames haveall been transmitted, then the process proceeds to block 37. In theexample shown in FIG. 5, after AC frame transmissions, at the end of theMU-TXOP, the remaining time is insufficient for transmitting a CF-Endframe. Note that frames belonging to secondary ACs may be longer orshorter than frames of the primary AC at each frame exchange sequence.Further, the terms “stream” and “path” need not be equivalent. Forexample, it is possible that an AP uses multiple streams to transmit toone STA.

According to an embodiment of invention, a TXOP is obtained by atransmitting wireless STA to be shared by multiple data frames, whichmay belong to different ACs. This allows multiple frames belonging todifferent ACs to be transmitted simultaneously. A MU-TXOP mechanismallows the owner of a TXOP (i.e., primary AC) to share the obtained TXOPwith other ACs that did not obtain the TXOP (i.e., secondary ACs). WhenMU-TXOP is used, ACs are classified into primary and secondary ACs.There are different backoff procedures for primary AC and secondary AC.

Events Types that Invoke Backoffs

According to conventional IEEE 802.11 standard, a backoff procedure isinvoked for an AC in a EDCAF-based channel access process when any ofthe following events occurs: \

-   -   Event a: A frame with that AC needs to be transmitted, wherein        the communication medium is busy as indicated by either physical        or virtual CS, and the backoff timer has a value of zero for        that AC.    -   Event b: The final transmission by the TXOP holder initiated        during the TXOP for that AC was successful.    -   Event c: The transmission of a frame of that AC fails, indicated        by a failure to receive a CTS in response to an RTS, a failure        to receive an ACK frame that was expected in response to a        unicast MPDU, or a failure to receive a BlockAck or ACK frame in        response to a BlockAckReq frame.    -   Event d: The transmission attempt collides internally with        another EDCAF of an AC that has higher priority, that is, two or        more EDCAFs in the same STA are granted a TXOP at the same time.

Backoff Procedures for Each Backoff Event Type in the IEEE 802.11Standard

-   -   Event a: Backoff is invoked and the value of CW[AC] is left        unchanged.    -   Event b: Backoff is invoked and the value of CW[AC] is reset to        CWmin[AC].    -   Event c and Event d: Both are failure events, wherein backoff is        invoked and the value of CW[AC] is updated as follows before        invoking a backoff procedure:        -   If the QSRC[AC] or the QLRC[AC] for the QoS STA has reached            dot11ShortRetryLimit or dot11LongRetryLimit respectively,            CW[AC] shall be reset to CWmin[AC];        -   Otherwise,            -   1) If CW[AC] is less than CWmax[AC], CW[AC] shall be set                to the value (CW[AC]+1)*2−1.            -   2) If CW[AC] is equal to CWmax[AC], CW[AC] shall remain                unchanged for the remainder of any retries.

A backoff timer is set to an integer value selected randomly with auniform distribution taking values in the range [0,CW[AC]] inclusive.All backoff slots occur following an AIFS[AC] period during which thewireless communication medium is determined to be idle for the durationof the AIFS[AC] period, or following an EIFS-DIFS+AIFS[AC] period,during which the wireless communication medium is determined to be idlefor the duration of the EIFS-DIFS+AIFS[AC] period, as appropriate.

Backoff procedures for MU-TXOP support according to embodiments of theinvention, summarized further above, are described below in more detail.

Backoff procedures are disclosed herein for the EDCA-based channelaccess mechanism based on MU-TXOP according to embodiments of theinvention.

Primary AC Backoff Process

The primary AC follows backoff rules as above for all backoff eventtypes. This is to maintain fairness among the transmitting wirelessstation and all other wireless stations in the BSS. When the primary ACtargets more than one destination wireless station with transmission ofdifferent spatial streams simultaneously, wherein certain transmissionssucceeded while others fail, successful transmission is defined as atleast one of the destination wireless stations successfully receivingsaid transmissions (i.e., transmitted information frames). Thetransmission is considered failed only if all destination wirelessstations failed to receive said transmissions.

In another embodiment, successful transmission is defined as the firstexpected acknowledgement of the initial downlink transmission in aseries of downlink transmissions (e.g. BA from STA-B) is received fromthe destination wireless stations successfully receiving saidtransmissions (i.e., transmitted information frames). The transmissionis considered failed only if the first expected acknowledgement of theinitial downlink transmission in a series of downlink transmissions isnot received.

Secondary AC Backoff Process

A backoff process for a secondary AC in response to each of theabove-mentioned backoff events, according to embodiments of theinvention, is described below.

In general, a secondary AC is provided an opportunity to share a TXOP(e.g., MU-TXOP) with the primary AC (a free ride) without negativeimpact on the operation of the primary AC. Further, a secondary ACshares the TXOP with the primary AC as long as it does not have anegative impact on its own transmit opportunity.

FIG. 6 shows a flowchart of a backoff process 60 for a secondary AC,according to an embodiment of the invention. In process block 61, thesecondary AC freezes its countdown timer when sharing a TXOP with theprimary AC. In process block 62, the secondary AC shares a TXOP with theprimary AC for transmitting its data frames. In process block 63, a backoff events occurs at secondary AC of the transmitting station. Inprocess block 64, the secondary AC determines the type of the backoffevent. In process block 65, at the end of the TXOP sharing, thecountdown timer of the secondary AC is either reset to a different valuefor each type of backoff event, or resumes by continuing its count downfrom where it left off before said TXOP sharing. In one embodiment,backoff processes for the primary and secondary ACs are implemented bythe MAC layer of the wireless stations (e.g., MU-TXOP module 16 in FIG.1).

Example backoff process implementations according to the process 60 fora secondary AC, in relation to each of said backoff events are describedbelow.

Secondary AC backoff process in response to backoff Event a:

-   -   A backoff Event a does not occur at a secondary AC because if        the wireless communication medium is busy, the primary AC will        not obtain an TXOP. If the communication medium is physically        busy, the transmitting station secondary AC should perform        backoff as in conventional procedure. If the communication        medium is virtually busy, the secondary AC checks if any other        AC of the transmitting station has obtained the TXOP and whether        sharing the TXOP is possible. If sharing of TXOP is granted to        the secondary AC, the secondary AC will then transmit and will        not invoke backoff. Otherwise, the secondary AC invokes backoff        as in the conventional procedure.

Secondary AC backoff process in response to backoff Event b:

-   -   A first example backoff process for a secondary AC in response        to a backoff Event b involves the secondary AC resuming backoff        timer countdown. This is because an opportunity to share TXOP of        another AC should not affect transmission by the secondary AC        itself.    -   A second example backoff process for a secondary AC in response        to a backoff Event b involves the secondary AC backing off with        CW reset to a predetermined minimum contention window value        CWmin. In this case, a new backoff timer value overwrites a        frozen backoff timer value before the TXOP sharing.    -   Because in this case the secondary AC is sharing the TXOP        obtained by the primary AC, its transmission of data frames is        limited by the length of TXOP obtained by the Primary AC. This        is because the sharing rule requires the time used to transmit        the frames of secondary AC cannot exceed the time used to        transmit the frames of the primary AC. Therefore, the length of        the shared TXOP may or may not fulfill the requirement of        transmitting what the secondary plans to transmit.    -   To facilitate understanding, assume the secondary AC plans to        transmit M frames if it gets its own TXOP. When sharing the TXOP        obtained by the primary AC, the secondary AC can transmit only N        frames. Also assume the N.sup.th frame was transmitted        successfully. There are two cases the transmitting station        secondary AC needs to consider.        -   Case 1: M<N        -   In this case, the secondary AC has finished transmission of            what it plans to transmit as if the TXOP was obtained by            itself. Therefore, the secondary AC invokes backoff as            routine.        -   Case 2: M>N        -   In this case, the secondary AC has not finished transmission            of what it plans to transmit as if the TXOP was obtained by            itself wherein the desired transmission is partially            finished. The AC resumes its transmission when it gets its            own TXOP or an opportunity of sharing again with other            primary TXOPs that will follow. To do this, the secondary AC            can freeze its original values of CW and backoff timer at            the start of the TXOP sharing and resume the count down            after the initial successful transmission.

Secondary AC backoff process in response to backoff Event c:

-   -   In this case, a transmission failure has just occurred. In a        first implementation of secondary AC backoff in response to a        backoff Event c, according to an embodiment of the invention,        the secondary AC checks whether it is the only AC that        experienced a transmission failure.        -   Case 1: If the secondary AC is not the only AC that            experienced a transmission failure, more than one AC that            shared the same TXOP experienced a transmission frame            collision. Therefore, the collision is in the vicinity of            the transmission sender wireless station. To avoid further            collisions, the secondary AC requests to backoff, together            with other ACs that experienced the transmission failure.        -   Case 2: If the secondary AC is the only AC that experienced            a transmission failure, the transmission failure only            occurred at one specific receiving wireless station. It is            possible that the collision only occurs in the spatial area            (vicinity) of the receiving wireless station. Because the            failed frame was transmitted using beamforming, a collision            occurring in one area does not mean it occurred in other            areas. Therefore, the transmission sender wireless station            can attempt a more aggressive procedure using DBB.        -   Using DBB, the AC can invoke backoff only in the direction            where the transmission failure occurred. For example, if            transmission to STA-D failed, the backoff can be marked only            for STA-D. For other directions where transmission failures            have not occurred, the AC can maintain its original values            of CW and backoff timer before the TXOP sharing and resume            the count down after the initial successful transmission by            sharing.        -   Referring to the example communication system 70 in FIG. 7,            two WLAN networks, BSS-1 and BSS-2, are operating with            certain coverage areas overlapped. Assume that in BSS-1 when            the AP STA-A is directionally transmitting a frame to STA-D,            at the same time in BSS-2 the AP STA-A′ is directionally            transmitting a frame to STA-C′. This leads to a collision in            the vicinity of STA-D and STA-C′ (as shown by “x” markings).            However, the frames directionally transmitted from STA-A to            STA-B and STA-C in BSS-1 will not be affected by the            collision of STA-D and STA-C′ frames.        -   Therefore, in BSS-1 only the AC associated with STA-D            requires backoff invocation. Later, when the AC gets it own            TXOP, if the TXOP is obtained when the original timer counts            down to zero, only frames destined to receiver STAs that did            not experience transmission failures in the previous TXOP            can be sent. If the TXOP is obtained when the            direction-specific timer counts down to zero, frames            destined to all receiver STAs can be sent, including the            ones that have experienced collision.    -   In a second implementation of secondary AC backoff in response        to a backoff Event c, according to an embodiment of the        invention, a first approach involves invocation of exponential        backoff, wherein the transmit opportunity of the secondary is        skipped. In a second approach, the transmission is stopped and        countdown resumed, wherein the AP can select another secondary        AC to share the TXOP with, or it can select a different receiver        station within the same secondary AC to share the TXOP with.

Secondary AC backoff process in response to backoff Event d:

-   -   In an implementation of secondary AC backoff in response to a        backoff Event d, the sending wireless station checks whether an        internal collision can be resolved by TXOP sharing. If the        internal collision cannot be resolved by TXOP sharing, then        exponential backoff is invoked. If the internal collision can be        resolved by TXOP sharing, the AP (coordinator) decides whether        to invoke backoff based on the transmission status, success or        failure, as described in the backoff procedures for Events b and        c, respectively, according to the invention. TXOP sharing may        not be possible when, for example, two transmitting station ACs        are targeting at the same receiver station, or strong        interference exists between two spatial streams to the two        receiver stations.

As is known to those skilled in the art, the aforementioned examplearchitectures described above, can be implemented in many ways, such asprogram instructions for execution by a processor, as software modules,microcode, as computer program product on computer readable media, aslogic circuits, as application specific integrated circuits, asfirmware, as consumer electronic devices, etc., in wireless devices, inwireless transmitters/receivers, in wireless networks, etc. Further,embodiments can take the form of an entirely hardware embodiment, anentirely software embodiment or an embodiment containing both hardwareand software elements.

FIG. 8 is a high level block diagram showing an information processingsystem comprising a computer system 100 useful for implementingdisclosed embodiments. The computer system 100 includes one or moreprocessors 101, and can further include an electronic display device 102(for displaying graphics, text, and other data), a main memory 103(e.g., random access memory (RAM)), storage device 104 (e.g., hard diskdrive), removable storage device 105 (e.g., removable storage drive,removable memory module, a magnetic tape drive, optical disk drive,computer readable medium having stored therein computer software and/ordata), user interface device 106 (e.g., keyboard, touch screen, keypad,pointing device), and a communication interface 107 (e.g., modem, anetwork interface (such as an Ethernet card), a communications port, ora PCMCIA slot and card). The communication interface 107 allows softwareand data to be transferred between the computer system and externaldevices. The system 100 further includes a communications infrastructure108 (e.g., a communications bus, cross-over bar, or network) to whichthe aforementioned devices/modules 101 through 107 are connected.

Information transferred via communications interface 107 may be in theform of signals such as electronic, electromagnetic, optical, or othersignals capable of being received by communications interface 107, via acommunication link that carries signals and may be implemented usingwire or cable, fiber optics, a phone line, a cellular phone link, anradio frequency (RF) link, and/or other communication channels. Computerprogram instructions representing the block diagram and/or flowchartsherein may be loaded onto a computer, programmable data processingapparatus, or processing devices to cause a series of operationsperformed thereon to produce a computer implemented process.

Embodiments have been described with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products. Each block of such illustrations/diagrams, orcombinations thereof, can be implemented by computer programinstructions. The computer program instructions when provided to aprocessor produce a machine, such that the instructions, which executevia the processor create means for implementing the functions/operationsspecified in the flowchart and/or block diagram. Each block in theflowchart/block diagrams may represent a hardware and/or software moduleor logic. In alternative implementations, the functions noted in theblocks may occur out of the order noted in the figures, concurrently,etc.

The terms “computer program medium,” “computer usable medium,” “computerreadable medium”, and “computer program product,” are used to generallyrefer to media such as main memory, secondary memory, removable storagedrive, a hard disk installed in hard disk drive, and signals. Thesecomputer program products are means for providing software to thecomputer system. The computer readable medium allows the computer systemto read data, instructions, messages or message packets, and othercomputer readable information from the computer readable medium. Thecomputer readable medium, for example, may include non-volatile memory,such as a floppy disk, ROM, flash memory, disk drive memory, a CD-ROM,and other permanent storage. It is useful, for example, for transportinginformation, such as data and computer instructions, between computersystems. Computer program instructions may be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

Furthermore, the computer readable medium may comprise computer readableinformation in a transitory state medium such as a network link and/or anetwork interface, including a wired network or a wireless network, thatallow a computer to read such computer readable information. Computerprograms (i.e., computer control logic) are stored in main memory and/orsecondary memory. Computer programs may also be received via acommunications interface. Such computer programs, when executed, enablethe computer system to perform the features as discussed herein. Inparticular, the computer programs, when executed, enable the processormulti-core processor to perform the features of the computer system.Such computer programs represent controllers of the computer system.

Though the embodiments have been described with reference to certainversions thereof; however, other versions are possible. Therefore, thespirit and scope of the appended claims should not be limited to thedescription of the preferred versions contained herein.

What is claimed:
 1. A method of wireless communication in a wirelesscommunication system, comprising: maintaining data blocks at a wirelesstransmitting station for transmission to multiple wireless receivingstations over a wireless communication medium, wherein the data blocksare organized based on access categories; each access categorycontending for a transmission opportunity period for access to thewireless communication medium; and upon successful contention, duringthe transmission opportunity period, transmitting a data block of aprimary access category from the transmitting station to a primarydestination wireless receiving station, simultaneously transmitting adata block of a secondary access category from the transmitting stationto a secondary destination wireless receiving station, over the wirelesscommunication medium; wherein contending for the transmissionopportunity period comprises each access category contending for accessto the wireless communication medium, and the secondary access categoryselectively invoking communication medium access backoff based on abackoff event, wherein invoking communication medium access backoffcomprises: in response to sharing a transmission opportunity periodbeing not possible, the secondary access categories invokingcommunication medium access backoff.
 2. The method of claim 1, whereininvoking communication medium access backoff comprises: in response to abackoff event, performing direction-based backoff for directionaltransmissions by identifying a collision spatial area and only invokingbackoff for transmissions directed to that spatial area.
 3. The methodof claim 1, wherein: invoking communication medium access backoffcomprises: a secondary access category freezing a countdown timer whenit begins to share the transmission opportunity period with a primaryaccess category; after sharing a transmission opportunity periodobtained by another access category, the secondary access categoryresuming countdown of the frozen backoff timer.
 4. The method of claim3, further comprising: resetting the backoff timer based on the backoffevent type before resuming communication channel access and backofftimer countdown, after sharing said transmission opportunity period. 5.The method of claim 3, further comprising: resuming communicationchannel access and backoff timer countdown, after sharing saidtransmission opportunity period.
 6. The method of claim 1, whereincommunication medium access backoff further comprises: in response to asuccessful transmission by a transmission opportunity holder initiatedduring the transmission opportunity period for an access category, thesecondary access category resuming backoff timer countdown.
 7. Themethod of claim 1, wherein communication medium access backoff furthercomprises: in response to a successful transmission by a transmissionopportunity holder initiated during the transmission opportunity periodfor an access category, the secondary access category backing off with acontention window reset to a predetermined minimum contention windowvalue, such that a new backoff timer value overwrites a frozen backofftimer value before the transmission opportunity period sharing.
 8. Themethod of claim 1, wherein communication medium access backoff furthercomprises: in response to an internal channel access attempt collisionat the transmitting station due to a higher priority access category, ifthe internal collision cannot be resolved by sharing a transmissionopportunity period, then invoking exponential backoff, otherwise thesecondary access category shares the transmission opportunity period fortransmitting data frames and invoke backoff depending on the results ofsaid transmission.
 9. The method of claim 1, wherein: the data blocksare organized into access categories in order of transmission priority;the transmitting station comprises a multiple-input-multiple-output(MIMO) wireless station, wherein the transmission opportunity comprisesa multi-user transmission opportunity (MU-TXOP) for simultaneoustransmission of data blocks at different transmission priorities duringthe MU-TXOP, over the wireless communication medium.
 10. The method ofclaim 1, wherein: transmitting during the transmission opportunityperiod comprises directionally transmitting the data blocks of theprimary access category from the transmitting station to the primarydestination receiving station over the wireless communication mediumwhile simultaneously transmitting the data blocks of the secondaryaccess category from the transmitting station to the secondarydestination receiving station over the wireless communication medium.11. The method of claim 1, wherein: contending for a transmissionopportunity period comprises performing Enhanced Distributed ChannelAccess (EDCA) to provide Quality of Service (QoS) for a data block inthe primary access category; and transmitting during the transmissionopportunity period comprises directionally transmitting the data blockof the primary access category from the transmitting station to theprimary destination wireless receiving station over the wirelesscommunication medium, and simultaneously transmitting data blocks ofsecondary access categories from the transmitting station to secondarydestination wireless receiving stations over the wireless communicationmedium.
 12. A wireless station for wireless communication in a wirelesscommunication system, comprising: a communication physical layerconfigured for wireless communication over a shared wirelesscommunication medium; and a channel access module configured formaintaining data blocks at a wireless transmitting station fortransmission to multiple wireless receiving stations over a wirelesscommunication medium, wherein the data blocks are organized based onaccess categories; wherein upon successful contention by each accesscategory for a transmission opportunity period for access to thewireless communication medium, during the transmission opportunityperiod the channel access module utilizes the physical layer to transmita data block of a primary access category to a primary destinationwireless receiving station, while simultaneously transmitting a datablock of a secondary access category to a secondary wireless receivingstation over the wireless communication medium; wherein contention forthe transmission opportunity period comprises each access categorycontending for access to the wireless communication medium based, andthe secondary access category selectively invoking communication mediumaccess backoff based on a backoff event, wherein in response to sharinga transmission opportunity period being not possible, the secondaryaccess category invokes communication medium access backoff.
 13. Thewireless station of claim 12, wherein said secondary access categoryinvokes communication medium access backoff in response to atransmission failure event, by performing direction-based backoff fordirectional transmissions by identifying a collision spatial area andonly invoking backoff for transmissions directed to that spatial area.14. The wireless station of claim 12, wherein: said secondary accesscategory invokes communication medium access backoff by freezing acountdown timer when it starts to share the transmission opportunityperiod with a primary access category, and after sharing a transmissionopportunity period obtained by another access category, the secondary ACresuming countdown of the frozen backoff timer.
 15. The wireless stationof claim 14, wherein said secondary access category resets the backofftimer based on the backoff event type before resuming communicationchannel access and backoff timer countdown, after sharing saidtransmission opportunity period.
 16. The wireless station of claim 14,wherein said secondary access category resumes communication channelaccess and backoff timer countdown, after sharing said transmissionopportunity period.
 17. The wireless station of claim 12, wherein inresponse to a successful transmission by a transmission opportunityholder initiated during the transmission opportunity period for anaccess category, the secondary access category resumes backoff timercountdown.
 18. The wireless station of claim 12, wherein in response toa successful transmission by a transmission opportunity holder initiatedduring the transmission opportunity period for an access category, thesecondary access category backs off with a contention window reset to apredetermined minimum contention window value, such that a new backofftimer value overwrites a frozen backoff timer value before thetransmission opportunity period sharing.
 19. The wireless station ofclaim 12, wherein in response to an internal channel access attemptcollision at the transmitting station due to a higher priority accesscategory, if the internal collision cannot be resolved by sharing atransmission opportunity period, the access category invokes exponentialbackoff, otherwise the secondary access category shares the transmissionopportunity period for transmitting data frames and invokes backoffdepending on the results of said transmission.
 20. A wirelesscommunication system, comprising: a wireless station; and wirelessreceivers; the wireless station comprising: a communication physicallayer configured for wireless communication over a shared wirelesscommunication medium; and a channel access module configured formaintaining data blocks at a wireless transmitting station fortransmission to the wireless receivers over a wireless communicationmedium, wherein the data blocks are organized based on accesscategories; wherein upon successful contention by each access categoryfor a transmission opportunity period for access to the wirelesscommunication medium, during the transmission opportunity period thechannel access module utilizes the physical layer for transmitting adata block of a primary access category to a primary destinationwireless receiver, while simultaneously transmitting data blocks of asecondary access category to a secondary wireless receiver over thewireless communication medium; wherein contention for the transmissionopportunity period comprises each access category contending for accessto the wireless communication medium based, and the secondary accesscategory selectively invoking communication medium access backoff basedon a backoff event, and wherein in response to sharing a transmissionopportunity period being not possible, the secondary access categoryinvokes communication medium access backoff.
 21. The system of claim 20,wherein said secondary access category invokes communication mediumaccess backoff in response to a backoff event, by performingdirection-based backoff for directional transmissions.
 22. The system ofclaim 21, wherein said secondary access category performs directionbased backoff by identifying a collision spatial area and only invokingbackoff for transmissions directed to that spatial area.
 23. The systemof claim 20, wherein: said secondary access category invokescommunication medium access backoff by freezing a countdown timer whenit starts to share the transmission opportunity period with a primaryaccess category, and after sharing a transmission opportunity periodobtained by another access category, the secondary access categoryresuming countdown of the frozen backoff timer.
 24. The system of claim23, wherein said secondary access category resets the backoff timerbased on the backoff event type before resuming communication channelaccess and backoff timer countdown, after sharing said transmissionopportunity period.
 25. The system of claim 23, wherein said secondaryaccess category resumes communication channel access and backoff timercountdown, after sharing said transmission opportunity period.
 26. Thesystem of claim 20, wherein a wireless receiver is configured to receivedata blocks transmitted from the wireless station over a wirelesscommunication medium, wherein the data blocks are organized based onprimary and secondary access categories, and extract from the receiveddata blocks, data blocks of the primary access category and process thedata blocks of the primary access category.
 27. The system of claim 20,wherein in response to a successful transmission by a transmissionopportunity holder initiated during the transmission opportunity periodfor an access category, the secondary access category resumes backofftimer countdown.
 28. The system of claim 20, wherein in response to asuccessful transmission by a transmission opportunity holder initiatedduring the transmission opportunity period for an access category, thesecondary access category backs off with a contention window reset to apredetermined minimum contention window value, such that a new backofftimer value overwrites a frozen backoff timer value before thetransmission opportunity period sharing.
 29. The system of claim 20,wherein in response to an internal channel access attempt collision atthe transmitting station due to a higher priority access category, ifthe internal collision cannot be resolved by sharing a transmissionopportunity period, the access category invokes exponential backoff,otherwise the secondary access category shares the transmissionopportunity period for transmitting data frames and invoke backoffdepending on the results of said transmission.